Immunology and Microbiology

Host-Microbe Interactions

Non-specific (Innate) Immunity

First line of defense (Non-specific resistance)• Physical and Chemical Defenses• Normal FloraSecond line of defense (Non-specific resistance)• Phagocytosis• Inflammation• Fever• Antimicrobial substances: Complement,

InterferonThird line of defense (Specific Resistance)• Antibodies• B cells and T cells

• Detect enemy

• Immobilize/kill enemy• Send out alarm• Recruit help• Make environment

unpalatable for enemy/prevent spread

• (process repeats)

• Toll-like receptors on macrophages; complement (3 ways)

• Phagocytosis; lysis• Cytokine release

• Inflammation; fever

Toll-like receptors: What are some evolutionarily conserved features of microbes that our immune cells could recognize?

Step 2.

Steps in phagocytosis

http://www.cellsalive.com/mac.htm

Clinical and laboratory features of patients with an inherited deficiency of neutrophil membrane complement receptor type 3 (CR3) and the related membrane antigens LFA-1 and p150,95. Ross GD.

Over the last 3 years a group of more than 20 patients has been described worldwide who have a similar history of recurrent bacterial infections and an inherited deficiency of three related leukocyte membrane surface antigens known as CR3, LFA-1 and p150,95 (function unknown). It is believed that the patients with this disease have a reduced or absent expression of all three antigen family members on different WBC types. Neutrophils have a reduced phagocytic response to bacteria and yeast as well as a reduced ability to migrate into sites of infection.

• Adapted from J Clin Immunol. 1986 Mar;6(2):107-13.

The Complement System

• Activated in 3 ways– Antibody-antigen (augments specific defense)– Molecules that recognize bacterial sugar polymer

(mannan)– “Random binding” to cell surfaces (C3b)

• Triggers a cascade• 3 effects

Effects of complement system

1. Opsonization

2. Membrane Attack Complex (Lysis)

Are G+ or G-ve cells more susceptible to killing by MAC? Why?

Bacterial cell wall structure

Gram-negative and gram-positive bacteria

http://www.blackwellpublishing.com/trun/artwork/Animations/Overview/overview.html

3. Inflammation

• What else triggers inflammation? Tissue damage

• What are features of inflammation?

• What is the inflammatory process?

Which of the following statements about inflammation is false?

A. Vasodilation results in leakage of blood components

B. The process can cause damage to host tissueC. It is always accompanied by a feverD. The signs of inflammation are redness, swelling,

heat and painE. It can be triggered by activating toll-like

receptors (macrophages produce cytokines)

INFLAMMATORY PROCESS

trauma or infectionvasodilation

chemotaxis and diapedesis

Is that pus?

Is inflammation a good thing?

The movement of phagocytes in the direction of an infection, due to attraction by complement, chemicals released by microorganisms, and the remnants of damaged cell membranes is a process called

A) phagocytosis.B) chemotaxis.C) diapedesis.D) cytoadherence.

• Hypothalamus controls body temp

• Pyrogens resets temp set point

• Pyrogens: cytokines; LPS

• Unfavourable for bacterial replication

• Favourable for immune response--phagocytosis; lymphocyte replication etc

FEVER

Interferon and viral infections: a protective alerting system

Complement and immunoglobulins can coat the outer surface of a microorganism, enhancing phagocytosis. This process is called

A) chemotaxis.B) fixation.C) opsonization.D) endotoxicity.E) membrane attack complexing.

Host-Microbe Interactions

1. Innate (Non-specific) Defensesa. Chemical and Physical;

(Biological)b. Normal Flora

Host-Microbe Interactions

1. Contact with microbe

a. colonization(normal flora)

b. defense breached c. Loss d. Allergy

2. Infection

a. cure, immunity b. establishment c. carrier

3. Disease

All of the following statements are true with regard innate immunity EXCEPT:

A)These responses are not directed specifically to the infectious agent.

B)These responses are not affected by prior exposure to the agent.

C)These responses are inborn.D)These responses are operating constantly to

prevent the establishment of any infection.E)All of the above are true with regard to

innate immunity.

First line of defense Barriers

Physical Chemical

An enzyme found in our tears, saliva, serum, and mucus that degrades the peptidoglycan of the cell wall of Gram-positive bacteria is called

A) amylaseB) lysozyme.C) keratinase.D) streptokinase.E) peptidase.

First line of defense-Chemical & Physical Barriers

Which of the following substances is produced by the cells in our body and interferes with the multiplication of viruses by stimulating the production of antiviral proteins?

A) antivirase.B) interferon.C) inhibitase.D) complement.E) multiplicase.

Which of the following are mechanisms that protect the respiratory system from infection?

1. mucus 2. mucociliary escalator 3. normal flora 4. lysozyme 5. acidic environment

A)1,2,5.B)1,3.C)1,2,3.D)2,4.E)1,2,3,4.

Normal Flora of Humans

• A normal human has approximately 1013 body cells and 1014 individual normal flora!

• microbes that grow on external and internal surfaces of the body without producing obvious harmful effects

• Transient microbial flora : only occasionally inhabit the body.

• Symbiotic relationships: Commensal, Mutualistic, Parasitic

What normal flora?

Body sites that harbor normal flora

Importance of the normal flora

• Prevent attachment of invading organisms• Produce antimicrobial substances against

other microbes that are pathogens• Stimulate immune system

– Cause the production of cross-reacting antibodies• Significant nutritional source of vitamins• Cause dental caries and gum disease

How do we acquire microflora?

• During birth & within first 12 hours after delivery

• Breast-fed v bottle-fed• Contact with people, environment, food.• Eruption of teeth & introduction of solid food.

What leads to changes in the normal flora?

• Antibiotic treatment• Immunosuppression• Diet• Changes in physiology , e.g. estrogen-glycogen

effect

Normal Skin Flora

• 1000 to >1 million/ cm2

• Diphtheroids: G+ rods & cocci, e.g. Corynebacterium & Proprionobacterium , e.g. P.acnes (acne)

• facultatively anaerobic, coagulase negative Staphylococci [carriers of coagulase positive S. aureus in population]

• Yeasts

First line of defense (Non-specific resistance)• Physical and Chemical Defenses• Normal FloraSecond line of defense (Non-specific resistance)• Phagocytosis• Inflammation• Fever• Antimicrobial substances: Complement, InterferonThird line of defense (Specific Resistance)• B cells and T cells• Antibodies and Humoral Response• Cellular Mediated Immunity

Genetic Immunodeficiencies

Common Variable Hypogammaglobulinemia: Affecting both males and females and occurring at any age, this disease is manifest by repeated pyrogenic infections. The B cells fail to mature to plasma cells. Passive Ig is the common treatment.

Chronic Candidiasis: Fungal infection by nonpathogenic yeast is the manifestation of a number of afflictions where few mature T cells are present. These patients often have B cells that respond to T-independent antigens, but Candida albicans (yeast) stimulates a poor antibody response.

1) What immune component fails to be made in CVH?2) How come a defect in T cells affects antibody response in chronic candidiasis?

The two arms of the adaptive immune system

Antigen: a molecule (or parts of one) that causes antibody generation

The specific region on an antigen recognized by an antibody

(Immunoglobulin)

Antibody StructureDiversity in antibodies due to variable region

An infinitely large number of possible immunoglobulins;5 different classes: IgG, IgM, IgA, IgD & IgEMade by V(D)J recombination http://en.wikipedia.org/wiki/V%28D%29J_reco

mbination

Fig. 16.06Effects of Antigen-Antibody Interactions

Effects of Antigen-Antibody Interactions-2

Parasites; virally – infected host cellsNK cells release perforinsand proteases (16.8)

How is the antibody response triggered?1. T-cell dependent antigens 2. T-cell independent Ags

e.g. polysaccharides, LPS

response of young children to these antigens is poor

Result: Clonal selection and expansion of B-cells

A plasma cell

Clonal Expansion

Memory

Result:

Why is the RER in plasma cells so extensive?

1. Negative selection

2. Affinity maturation

3. Class switching: IgM – IgG—IgG / IgA

4. Formation of memory cells

Fig. 16.11Memory Cells mediate secondary response and lifelong immunity

Cellular Immunity

1. Cytotoxic T cells (CD8+)• Eliminates cells infected with virus, intracellular parasite

2. Helper T cells (CD4+)• Mediates B-cell proliferation; macrophage activation

Both stimulated by dendritic cells (cells of innate immunity)Both produce cytokines that stimulate own proliferation

T cells activated by dendritic cells

A

B

C

A. Recognition of virally-infected cell by cytotoxic T cell results in apoptosis

B: Helper T-cell activation and interaction with B-cells

Fig. 16.19C. Helper T- cells can also activate macrophages

Activated macrophage: a hungry beast!

•enlarged•membrane becomes irregular•increased number of _lysosomes, containing antimicrobial substances________•produce nitric oxide

Applications of Principles of Immunity

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IgG levels in fetus vs infant

What is passive and what is active?

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Principles of Immunization

• Natural or artificial • Passive or active• Natural passive , e.g. IgG across the placenta • Artificial passive, use of immune serum

globulin e.g. IgG fraction of donor blood.• Active involves lymphocytes and confers

lasting protection due to memory

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Can you put the types of vaccines listed into categories?

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Vaccines

• Induce artificial active immunity• Preparation of living or inactivated microbe or

virus or their components.• Adjuvants help to induce better response• Effective vaccines should be safe, few side

effects, lasting protection, low cost, stable, easy to administer

• Should induce appropriate specific response

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Attenuated immunizing vaccines• Use modified live microbe/virus• Induce infection & mild disease and solid long lasting immunity.• Single dose can induce immunity• Potential for spread to other people helps to develop HERD IMMUNITY• Disadvantages: may cause disease, cannot use in pregnancy, require

refrigeration• Examples: measles, mumps, rubella, Sabin polio vaccine, (Vaccinia ( for

smallpox))

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Inactivated Immunization

• Inactivated by chemical treatment but still antigenic• Cannot cause infection• Disadvantages: require several boosters, may cause side

effects

• Whole agent--Use inactivated bacteria or virus – Examples : Salk polio vaccine, diphtheria & tetanus toxoids

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Inactivated Immunization: Subunit vaccines

• Use isolated antigens or antigen fragments: a subunit of the total agent– bacterial toxin (toxoid), protein subunit,

polysaccharide – e.g vaccines against meningococci, pneumococci,

pertussis, H. influenza– Recombinant vaccine, e.g. Hepatitis B. Require

several doses.

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Future developments & information

• HIV/AIDS, Malaria, cancer• Use of DNA alone• Further information: www.immunizationinfo.org vaccine.chop.edu (Children’s Hospital of Philadelphia) • Developing New Smallpox Vaccines, in EID, vol7, #6, 2001. On

line at www.cdc.gov/eid

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Serology

• Testing for the presence of a specific antigen using specific antibody (antiserum)

• Examples: ELISA blood test for HIV, home pregnancy testhttp://www.sumanasinc.com/webcontent/animations/content/ELISA.html

Mechanisms of Pathogenicity

Attachment and ColonizationAvoidance of Immune System

Toxin Production

• Washwater containing Pseudomonas was sterilized and used to wash cardiac catheters. Three patients undergoing cardiac catherization developed fever, chills, and hypotension. The water and catheters were sterile. Why did the patients show these reactions?

Host-Parasite Dynamics

1. Reservoirs2. Modes of transmission3. Adherence and invasion of host4. Growth in/on host5. Evade host defense6. Damage host7. Leave host for reservoir or new host

How do bacterial pathogens overcome host defenses?

VIRULENCE FACTORS

bacterial products or structural components that contribute to pathogenicity or disease

A. promote bacterial colonization of the host(infectivity and invasiveness)

B. obtain nutrients, especially ironC. evade immune systemD. actively harm the host (toxins)

1. Motility

Swim through mucus and towards epithelial cells of mucus membranes (chemotaxis) Most microbes that colonize bladder, intestinesare motile (need to reach epithelial cells before flushed out)

Non-motile Vibrio cholerae mutants are less virulent

A1. Adherence Factors: Attachment and Colonization

2. Pili and fimbrae with adhesins

Pili allow uropathogenic bacteria to resist flushing and colonize the urethra.

Once bacteria attach, some remain on cell surfacelike E. coli and biofilm bacteria.

Others INVADE host cells.

2a. Capsule

Polysaccharide coat is sticky and aids attachmentAlso protects cell from inflammatory responseReduced complement fixation and phagocytosis.

3b. Biofilm: community of encapsulated bacteria

Aid attachment of other bacteria; evade host immunesystem

A2. Invasion

Proteins that induce phagocytosis in non-phagocytic cells

Advantage of living in host cell:

Ready supply of nutrientsProtected environment

Bacterium escapes fromvacuole and lives in cytoplasm

Mechanism of Invasion: Type III secretion system

Pore formed through bacterial membrane and walland human cell membrane.

Deliver proteins directly from bacterial cytoplasmto human cell cytoplasm that polymerizeactin filaments.

Invasion virulence factors that damage tissueor dissolve materials

e.g. Collagenase breaks down connective tissueClostridium perfringens: gangrene

Invasion of tissues possible too.

C. Virulence Factors that help evade immune system.

i. Avoid or escape phagocytosis

• destroy phagocytes or signals for recruitment• capsule• type III secretion of proteins that directly inhibit phagocytosis: Yersinia pestis

•prevent opsonization (Protein A and Fc receptor)

Antigenic variability (phase variation):alter genes that encode surface proteins like adhesins

Low immunogenicity: ‘Teflon’ pathogens (spirochaetes) few surface proteins

IgA protease

ii) evade specific immune response (antibodies)

Viral evasion of cytotoxic T cell and NK cell attack

D. TOXINS: Virulence factors that damage the host.

Endotoxin is only released upon death (lysis) of bacterium

What causes lysis?

LPS, especially when in the blood, can cause a number of pathophysiological changes such as:

a. fever production b. inflammation c. tissue destructiond. respiratory distresse. capillary damage (leading to petechial rash, capillary leakage, and hypovolemia f. intravascular coagulation g. hypotension h. decreased cardiac outputi. Septic shock j. wasting of the body k. diarrhea (from endotoxin in intestines)

Exotoxins

Released from viable bacteriaGram-negative and gram-positive cellsMost potent toxic substances known

1 ng of botulinum toxin kills 1000 peopleBind to specific receptors on host cells(more localized effect than endotoxin)

•Neurotoxins (nervous tissue)•Cytotoxins (general tissue)•Enterotoxins (GI tract)

Effect depends on cell type the toxin binds

Examples of bacterial exotoxins

Botulinum toxin (neurotoxin)Blocks release of neurotransmitterParalysis results

Cholera toxin (enterotoxin)Fluid loss results

Genetics of Pathogenicity

can involve many genes, complex.

chromosomal, plasmids, bacteriophage

Transfer of virulence genes via transformation, transduction, and conjugation.

Virulence factor genes often transferred as groups called pathogenicity islands: type III secretion system